The breakdown characteristics of metal-AlN-metal structures are reported as a function of contact diameter. The bulk AlN was grown by a HVPE method, resulting in a resistivity of 4 x 10(8) Omegacm. Front-side contact diameters of 175-600 mum were fabricated, displaying breakdown voltages up to similar to6300 V at 25 degreesC. Breakdown appeared to initiate at internal surfaces related to grain boundaries or cracks in the material. The results indicate the great promise of the Al(Ga)N system for high power rectifiers. (C) 2002 Published by Elsevier Science Ltd.